Internal combustion engines require valve actuation systems to control the flow of combustible components, typically fuel and air, to one or more combustion chambers during operation. Such systems control the motion and timing of intake and exhaust valves during engine operation. In a positive power mode, intake valves are opened to admit fuel and air into a cylinder for combustion and exhaust valves are subsequently opened to allow combustion products to escape the cylinder. This operation is typically called a “positive power” operation of the engine and the motions applied to the valves during positive power operation are typically called “main event” valve actuation motions. Auxiliary valve actuation motion, such as motion that results in engine braking (power absorbing), may be accomplished using “auxiliary” events imparted to one or more of the engine valves.
Valve movement during main event positive power modes of operation is typically controlled by one or more rotating cams as motion sources. Cam followers, push rods, rocker arms and other elements disposed in a valvetrain provide for direct transfer of motion from the cam surface to the valves. The use of a valve bridge may impart motion to plural valves from a single upstream valvetrain. For auxiliary events, “lost motion” devices may be utilized in the valvetrain to facilitate auxiliary event valve movement. Lost motion devices refer to a class of technical solutions in which valve motion is modified compared to the motion that would otherwise occur as a result of actuation by a respective cam surface alone. Lost motion devices may include devices whose length, rigidity or compressibility is varied and controlled in order to facilitate the selective occurrence of auxiliary events in addition to, or as an alternative to, main event operation of valves.
Lash adjustment features are typically provided on valve actuation systems to facilitate the elimination of lash, which is excessive clearance between valvetrain components that can lead to excessive noise, vibration, impact forces and wear. For example, during braking events, substantial lash may be introduced into the engine valvetrain. Lash adjusters, which are typically hydraulic lash adjusters (“HLA's”), may include mechanical components that cooperate to expand under hydraulic pressure in a lash take-up mode during one portion of the valve cycle, typically when the valvetrain is under low load or unloaded, and then assume a hydraulically “locked” or incompressible mode during another portion of the valve cycle, typically when the valvetrain is under high load, for example, during a main event actuation. One challenge related to the use of HLA's is the prevention of over-extension or “jacking” of the HLA, which may occur when the HLA is permitted to extend too far in the take-up mode and becomes hydraulically locked in the over-extended position. This can result in excessive valvetrain forces and other undesirable consequences. As such measures have been taken in the prior art to prevent jacking by maintaining suitable loads on the HLA or to limit HLA extension.
Prior art valve actuation systems with lash adjustment include systems such as those described in U.S. Pat. No. 9,611,767, and US patent Published Application No. 2015/0354418. The subject matter of both of these documents is incorporated herein by reference in its entirety. Such systems provide lash adjustment features that may be integrated into valvetrain components in a fulcrum valve bridge arrangement. In such systems, the lost motion components are typically situated in the same load path as the HLA. These systems have been developed and are particularly suited for valve actuation systems that utilize a dedicated cam for auxiliary motion. However, their application to other types of valve actuation systems, such as systems which utilize lost motion cams rather than dedicated auxiliary cams, may introduce complexities with regard to the prevention of HLA jacking.
In contrast to dedicated cam auxiliary motion systems, which utilize a dedicated cam for imparting auxiliary motion, lost motion cam systems typically use at least one cam with different profiled lift sections on the same cam lobe to impart motion for respective main event and one or more auxiliary events. These different profiled lift sections are activated or deactivated using a separate lost motion mechanism, such as a piston or actuator, located in the valvetrain. Example auxiliary events include engine braking, early exhaust valve opening (EEVO), or late intake valve closing (LIVC) lift events, and can be imparted to one or more valves in a valve set (i.e., two exhaust valves for a respective cylinder). Lost motion auxiliary valve lift systems, such as lost motion braking systems may employ a single rocker associated with the lost motion cam and a valve bridge associated with the rocker for actuating two engine valves in main event motion. Auxiliary valve lift or braking motion on one of the valves is facilitated by an auxiliary valve lift or braking actuator, which is a lost motion device that may be housed in the rocker and may selectively impart auxiliary or braking motion to the valve by way of a bridge pin disposed in the bridge and providing for independent motion relative thereto. The auxiliary valve lift or braking actuator is selectively activated and deactivated such that the auxiliary or braking event lift profile section or lobe on the lost motion cam only results in auxiliary or braking motion on the valve when an auxiliary event, such as engine braking is desired.
Adaptation of prior art systems, such as those described above, to lost motion cam environments, must factor in a number of considerations if they are to optimally support operation. For example, in systems that utilize a lost motion cam and a single rocker, often with an inboard valve actuator, for both main event and auxiliary (braking) valve motion, the interplay (i.e., fulcrum ratio and rocker ratio) between the rocker and fulcrum bridge may be different for main and auxiliary events. In addition, the primary (main event) motion is a higher lift event than the secondary (auxiliary or braking) motion lift events. Given these circumstances, the use of an HLA and a lost motion assembly on the same load path as in prior art systems may introduce complexities with regard to the interplay of those components. More particularly, the higher lift event may result an extension of the HLA beyond a tolerable limit suitable for a subsequent secondary motion, leaving the HLA in jacked or pumped out condition that increases risk of improper valve motion during the auxiliary event. Conversely, during auxiliary motion events operating on a single valve, the HLA may extend to take up a gap in the main event load path. This extension may result in improper valve motion during a subsequent main event motion. Prior art systems have provided limiters on the HLA piston stroke to ensure that the piston will not overextend. However, this requires lash to be set on the auxiliary valve motion system to ensure that the appropriate lift is achieved.
It would therefore be advantageous to provide systems that address the aforementioned shortcoming and others in the prior art.